Link Aggregation in accordance with IEEE 802.1AX-2008 (formerly IEEE 802.3ad) describes how to combine multiple LAN cables in parallel.

http://standards.ieee.org/about/get/802/802.1.html
see IEEE 802.1AX-2008 IEEE Standard for Local and Metropolitan Area Networks – Link Aggregation
for slow protocols, see http://standards.ieee.org/about/get/802/802.3.html section five, annexe 57A page 719

z/VM 1.4 HCPLCPDU DSECT ! http://www.vm.ibm.com/pubs/cp540/LCPDU.HTML clear info on structure
syskonnect http://cs.uccs.edu/~scold/doc/linkage%20aggregation.pdf

prez several protocols http://fr.slideshare.net/TomChou/ethernet-protocol
paper on jumbo frames and lacp, to read (and validate by someone skilled ;) http://www.fragmentationneeded.net/2011/12/jumbo-frames-support-lacp-and.html